There is an interest in surface sensitive techniques for analyzing the amount of molecules and larger substances, their chemical and physical properties, and their interactions with other molecules or materials. Properties that are of interest are e.g. the concentration of molecules in a solution or gas, the surface concentration of molecules on a sensor surface, the reaction kinetics of interacting substances, the affinity of the substances, allosteric effects or epitope mappings. Examples of interacting substances are antigen-antibody, protein-protein, receptor-ligand, DNA-DNA, DNA-RNA, protein-DNA, peptides-proteins, carbohydrates-proteins, glycoproteins-proteins, etc. There is also an interest in measuring the concentration of different gases or liquids, which can be performed by measuring the change in the optical density of a sensing material, e.g. polymer films, that are affected by some substance, e.g. fluid or gas [1, 2]. Change of conformation or formation of new materials on a surface, e.g. blood coagulation and fibrinolysis are also of large interest [3] [4] [5] [6]. There are many techniques that are suitable for these tasks, e.g. surface plasmon resonance (SPR), resonant mirror, grating couplers, interferometers, surface acoustic wave (SAW), Quartz Crystal Microbalance (QCM) etc. SPR is a popular technique, which have been proven to be both sensitive and reliable.
Areas of application are e.g. measurement of concentration of substances in biological research, biochemistry research, chemical research, clinical diagnosis, food diagnostics, environmental measurements, etc. Kinetic measurements can be used to determine rate constants as (kon) and (koff). Affinity measurements can be used to determine equilibrium association (KA) or dissociation (KD) constant as well as avidity.
SPR is a well-known phenomenon that is a bound electromagnetic wave, due to oscillations of electrons at the interface between a plasma and a dielectricum. The surface plasmon can only exist at an interface between the plasma (e.g. a metal) and the dielectricum. A change in the optical constants of the dielectricum will change the propagation constant of the surface plasmon. The surface plasmon can be excited by light if the propagation constant of the light parallel to the interface is equal to, or close to, the propagation constant of the surface plasmon. Normally, the Kretschmann configuration [7] is used where a thin metallic film is applied on a prism, having a higher refractive index than the measured sample. This is also denoted backside illumination, because no light is propagating in the sample medium. The surface plasmon is then evanescently excited under total internal reflection, i.e. at an incident angle, normal to the surface, larger than the critical angle. At a certain incident angle, the component of the wave vector parallel to the surface meets the real part of the complex wave vector for a surface plasmon, and hence the light will couple into the surface plasmon and propagate at the interface between the plasma and the dielectricum. The surface plasmon will reradiate into the prism, and for a certain thickness of the plasma a destructive interference will occur, leading to zero or close to zero intensity of the reflected light. For a smooth surface of the plasma, coupled light will be absorbed in the plasma and generate heat. When molecules bind close to the interface (within the probe depth of the surface plasmon) the interaction can be detected by a shift in the resonance condition of the surface plasmon. This can be detected as a shift in a reflected light intensity. This is also the case when a layer changes its density due to conformal changes or external interference.
The most common way to design an SPR apparatus is to use a prism (triangular, hemispherical or an arbitrary shape) and apply to the prism a separate planar substrate carrying the SPR-metal. In this case it is necessary to use a refractive index matching material between the prism and substrate to obtain good optical coupling. The material can either be an opto-gel [8] or a refractive index matching fluid. The use of an opto-gel has the disadvantage of wear, optical imperfection and high cost. If a refractive index matching fluid is used, a circumstantial procedure of application and cleaning is needed, besides the extra cost. There are other configurations, e.g. a prism with evaporated metal film [9] and SPR-light-pipe [10], that do not need an optical coupling medium. Yet another configuration that doesn't need to use an optical coupling medium, is a capillary [11]. In another configuration, the fiber optic SPR [12-14], on can refrain from using an optical coupling medium, but for the case of an exchangeable probe, a coupling medium is needed.
The surface plasmon resonance (SPR) phenomenon was already described in 1959 [15] and SPR apparatuses for thin adlayer analysis have been thoroughly described since 1968 [16, 17]. SPR setups for biosensing were used for the first time in 1982 [1] and for imaging applications in 1987 [18, 19]. With imaging SPR, also denoted SPR microscopy, new applications arise, e.g., label free—real time—multi spot biochemical analyses [20, 21], which can increase the throughput tremendously. The pioneering work on imaging SPR was undertaken by Knoll et al., who investigated surfaces patterned with Langmuir-Blodgett films [22, 23]. They also investigated the physical aspects of the technique, including lateral resolution [24], and proposed different setups, e.g. the rotating grating coupler [25].
Most SPR setups utilize a separate planar sensing substrate, refractive index matching layer, and a coupling element, e.g. a prism. Using a planar sensor surface with multiple sensor areas arranged in a two-dimensional way, means that there are a couple of somewhat cumbersome (and expensive) ways to read the optical output from the surface plasmon resonance device. In principle, there are two methods to perform a readout, by mechanically scan the sensor substrate, or the use of imaging optics. Not only is the read out complex, but the distribution of samples for investigation is often very complex, with use of valves and channels or expensive autosamplers.
However, there are approaches that do not use planar substrates. Chinowsky et al. are using an approach [11], where a capillary tube is used, U.S. Pat. No. 6,480,282. The tube wall itself is the then the coupling medium, and no index matching fluid is necessary. The capillary is useful for multi sensor configuration, where the other techniques can be critical angle detection, fluorescence, chemiluminiscence, adsorption or Raman scattering. It can, with difficulties be used as an axial multispot sensor, and with severe difficulties may be with some sensors spots radially separated. However, this approach has several disadvantages. It is very difficult to cover the inside of the capillary tube with a metal of precise thickness, due to the small diameter and long length. Capillary tubes are not normally high precision optics, which will distort optical images. The capillary device is suitable for one or a few sensing spots, and the small diameter means that there will be difficulties to manufacture device with many sensors spots. The capillary device has typically a relatively large diameter, e.g. 400 μm, compared to a thin flowcell, e.g. 10 μm, making the capillary tube inefficient regarding small sample volumes, and mass transport.
Another structure similar to the capillary device is proposed by Nakaso Nobutaka, Japan patent JP2003-294616. It uses a curved cavity, with a diameter of typically 20 μm e.g. a cylinder, or part of a sphere, which is formed in a transparent block. The block has a typical dimension of 2×2 mm, and is typically cut from a glass wafer of thickness 0.2 mm. The different blocks, having different recognition molecules, can be stacked. The surface plasmon is exited radial inside the cavity.
Yet another structure that uses a curved sensor surface is proposed by Atsushi et al. Japan Patent JP2003-075333. This device uses curved cavities for recognition sites, preferably many cavities are used for a multi spot sensor. The cavity can be cylindrical, spherical or an arbitrary curvature, and the surface plasmon is exited radially, as described by Chinowsky. However, the outer surface, which is hit by the incident light, is planar. The proposed cavity is not intended for multiple sensor areas.
Using a small radius at the surface plasmon carrying surface, will not only lead to mismatch between wavevector for surface plasmon and incident light, but also leads to difficulties to obtain small light beams and smooth reflectance curves.
A SPR-setup utilizing a convex curved SPR-supporting surface is described by Rooney et al., Sensors and Actuators B, 26 Apr. 2006. There is also described a SPR-setup consisting of a tubular cup, where a SPR-supporting layer is present on the planar bottom, EP 1186881, Haya et al., 2002 (Fuji Photo Film Co).